The present disclosure is directed to a protective device which protects wireline supporting logging tools from breaking the wireline by excessive pulling on the wireline. It is used in circumstances in which snagging of the cable supported load might accidentally break the cable and risk loss of the cable supported sonde.
Assume that a well has been drilled to a specified depth and has an uncased portion. In that region, there are areas where a protruding, irregular formation in the borehole may snag the logging cable or tool supported thereby. In addition, the tool may be supported on a coil tubing for lowering or raising the well borehole. Ordinarily, this occurs during raising of the tool in the borehole where the logging cable is maintained in tension. Snagging may increase the tension on the cable and thereby break the cable, dropping the logging tool and requiring an expensive fishing job to retrieve the sonde. Of course, the cable will have to be repaired also.
Assume for purposes of illustration that the logging cable is to support a first tool and then what tool is switched, and a second tool is installed. The tools may not weigh the same. Assume for descriptive purposes that one tool weights 200 pounds while the other tool weights 400 pounds. When weight variations are encountered the loading on the cable is different. The present apparatus is a overload protection device which guards against overloading the cable notwithstanding changes in the weight of the sonde affixed to the cable. The present apparatus is a device which connects between the cable and the sonde and which is adjustable to accommodate variations in tool weight as exemplified above, and relates that ability while also providing an adjustable range of loading imparted to the cable resulting from snagging. Assume for purposes of discussion that the cable can tolerate additional loading of 200 pounds above the weight of the tool. In that event, the tool when snagged can be pulled free provided the cable tension does not exceed 600 pounds with a 400 pound tool. If the load is greater than 600 pounds, damage might result. It is difficult to measure cable stress solely by what occurs at the surface. At the surface, the operator is normally equipped with a cable spool or drum with a motor (usually hydraulic) which rotates that spool at a specified rate. The surface located sheave is supported by a load cell monitored structure which cell output is normally indicative of cable tension. However, that is misleading becasue cable tension at the surface is not the same as cable tension at the tool in the borehole. When a cable is broken, it often occurs at the sheave. That is, the cable typically is broken somewhere between the ends. Normally, the cable connector is made to be the weak link in the connective equipment because separation at the cable connector is preferable to parting the cable. When the tool is snagged and held a short moment during retrieval, the cable might break at the sonde where separation is desirable.
The present apparatus is a cable head permitting connection of the cable to the sonde and which provides a signal when axial loads on the cable become excessive. This cable load value is adjustable to accommodate an adjustale peak load setting for snagging and is also adjustable to take into account changes in sonde weight. These adjustments enable a system to be devised which forms an output signal indicative of exceeding an adjustable but specified cable load. This signal is relayed to the surface and permits the operator to reduce cable tension, and thereby protects the cable, sonde and connective cable head therebetween from rupture in the event of excessive pull.
The output of the device is a signal on an electrical conductor through the device which conductor extends along the logging cable to furnish a surface output signal for the operator.